1. Field of the Invention
The present invention relates to a method for connecting optical fibers, and a heat treatment apparatus to be used therefor. More particularly, the present invention relates to a method for connecting end faces of optical fibers of different kinds each having a different mode field diameter (MFD) with small loss and in a stable state, and it also relates to a heat treatment apparatus which is effective when the different optical fibers are used at the same time.
2. Prior Art
Recently, a larger transmission capacity in optical communication systems has been demanded. To cope with this demand, investigations about dispersion management lines have been conducted.
An example of a dispersion management line L0 is shown in FIG. 1.
The line L0 is constituted by connecting at a connection point J0 end faces of a single mode optical fiber 1 having a positive dispersion characteristic shown in FIG. 2 and a dispersion compensating optical fiber 2 having a negative dispersion characteristic shown in FIG. 3. Optical amplifiers 3, 3 are connected to the optical fiber 1 and the optical fiber 2 respectively.
The dispersion management line L0 has, after all, the dispersion characteristic shown in FIG. 4 and is capable of transmitting, at high speed, light, for example, of a wavelength of 1550 nm in bandwidth, which is suitable for use in a wavelength division multiplexing (WDM) transmission method that enables long distance transmission and large capacity transmission. This dispersion management line L0 is ready to be used in submarine optical cables, for example.
As the single mode fiber (SMF) 1 to be used for the dispersion management line L0, there is a 1300 nm zero dispersion optical fiber. As the dispersion compensating optical fiber 2, there are dispersion compensating fiber (DCF), dispersion slope compensating fiber (DSCF), and reverse dispersion fiber (RDF), for example.
In the case of the above-illustrated 1300 nm zero dispersion optical fiber which is the SMF, its core is formed of silica doped with GeO2, and its cladding is formed of pure silica. The MFD on a wavelength of 1550 nm is 9 to 11 μm. In the SMF with the enlarged MFD, the MFD is 11 μm or more.
In the case of the dispersion compensating optical fiber having the negative dispersion characteristic, its core is formed of silica doped with, for example, a high concentration of GeO2, and its cladding is formed of silica doped with fluorine, as a refractive index difference needs to be a high value of approximately 3%. Its core diameter is about 2 to 3 μm, which is extremely small compared with the core diameter of the SMF. The MFD on the wavelength of 1550 nm has a value of about 5 μm. That is, the dispersion compensating optical fiber has the core diameter and MFD that are smaller compared with the SMF.
Therefore, when the end faces of the above two optical fibers of different kinds are simply fusion-spliced, even if their optical axes are adapted, connection loss is caused by the difference of the MFDs at the connection point J0, which further causes an optical leakage. For example, when the optical axis of the SMF with its MFD of 10 μm and that of the dispersion compensating optical fiber with its MFD of 5 μm are merely adapted and fusion-spliced, the connection loss in the fusion-spliced portion would be about 1.94 dB.
In such a case, a TEC method (Thermally Defused Expanded Core Method) is usually applied to reduce the occurrence of the connection loss in the fusion-spliced portion.
In the TEC method, after the end faces of the optical fibers of different kinds are fusion-spliced, heat treatment is applied to the fusion-spliced portion thus formed, so that dopant in the core is diffused to the cladding to substantially enlarge the core and the MFD.
For example, the TEC is applied to the fusion-spliced portion of the SMF and the dispersion compensating optical fiber. Since a softening temperature of the cladding of the dispersion compensating optical fiber (fluorine-doped) is lower than that of the cladding of the SMF (pure silica), as to the speed at which the dopant (GeO2) in the cores of both optical fibers diffuses to each cladding, it is faster in the dispersion compensating optical fiber than in the SMF. In this way, in the process of heat treatment, the dopant in the core of the dispersion compensating optical fiber diffuses preferentially, and the core diameter will be substantially enlarged at the fusion-spliced portion to correspond to the core diameter of the SMF. That is, the correspondence of the MFDs reduces drastically the connection loss between the two optical fibers.
Now, the above-described heat treatment that has been conventionally performed will be described.
First, an example of the heat treatment using burner flame will be described in accordance with FIG. 5.
For example, a fusion-spliced portion X is formed by fusion-splicing the end face of the MFD-enlarged SMF 1 and that of the dispersion compensating optical fiber 2 to make one connection line L0.
One end of the connection line L0 is connected to a light source 4, and the other of which is connected to a power meter 5. Then, the fusion-spliced portion X of the connection line L0 is disposed in a heat treatment apparatus 6, and the fusion-spliced portion X is heated with the burner flame.
In this event, light is made incident on the connection line L0 from the light source 4. The power meter 5 receives the light, and keeps measuring optical loss in an optical path from the light source 4 to the power meter 5 at every moment. On the basis of the varying measured values, the connection loss in the fusion-spliced portion X is calculated at every moment. At the time when the calculated connection loss is less than a target value of the connection loss, the burner flame heating is stopped.
Next, the heat treatment using an electric discharge will be described in accordance with a flowchart shown in FIG. 6.
First, two optical fibers of different kinds are set in a discharge heat treatment apparatus so that a main discharge is applied to them, and the two optical fibers are fusion-spliced (step S1).
Then, an image of the formed fusion-spliced portion is picked up by a camera. Furthermore, image processing is applied to the image. A plurality of pieces of information such as a luminance distribution in the fusion-spliced portion are measured (step S2).
In step S3, on the basis of the plurality of pieces of information obtained in step S2, the connection loss in the fusion-spliced portion is calculated.
In step S4, the calculated value obtained in step S3 and the target value of the connection loss are compared. When the calculated value is judged to be larger than the target value, a signal indicating that an additional discharge is needed is output to step S5 to actuate the discharge heat treatment apparatus. When the calculated value is judged to be smaller than the target value, a signal to stop the discharge treatment is output to the discharge heat treatment apparatus.
Discharge conditions in the heat treatment after step S2 are decided on the basis of information such as each MDF of optical fibers of different kinds, the calculated value of the connection loss, and the target value of the connection loss.
After the additional discharge in step S5, operations after step S2 are repeated. At the time when the calculated value of the connection loss is judged to be equal to or smaller than the target value in step S4, the discharge heat treatment is stopped.
The described two kinds of heat treatment both calculate the connection loss in the fusion-spliced portion and compare the calculated value and the target value of the connection loss to decide the time to stop the heat treatment.
However, as the both types of the heat treatment calculate the connection loss on the basis of the plurality of pieces of information, there is a problem that the calculated value is not always accurate.
This makes it impossible to finish the heat treatment at an appropriate timing in some cases, which leads to an insufficient heat treatment or conversely an excessive heat treatment. This might cause the connection loss in the connection line that connects the end faces of the optical fibers of different kinds to be larger than the target value.
For such a problem, it is considered to be effective to use a method of directly measuring the connection loss in the fusion-spliced portion with an OTDR.
There are some cases, however, where the connection loss in the fusion-spliced portion can not be measured directly even with the OTDR.
For example, there is a case of the submarine optical cable. The submarine optical cable is normally constituted of many optical cables connected via repeaters. In the process of laying the submarine optical cables, while laying the submarine optical cables on the sea bottom from a vessel, the optical cables are interconnected and the optical cables and the repeaters are connected on the vessel.
In this case, it is possible to form the connection line by fusion-splicing the optical fibers of different kinds.
It is, however, impossible to connect the connection line with the OTDR at the time of the heat treatment after the fusion splice. Therefore, it is substantially impossible to directly measure the connection loss. The above connection line can be connected neither with the OTDR nor the light source and the power meter that have been described earlier.
Therefore, as to the measurement of the connection loss in the fusion-spliced portion of the submarine optical cables, there is no way but to use the described method of image processing to measure the connection loss. However, the image processing does not always provide an accurate measured value of the connection loss. It is for this reason very difficult to decide the appropriate time to terminate the heat treatment. As a result, the insufficient heat treatment or excessive heat treatment often causes the connection loss to be larger than the target value.
Another example is the case of an optical amplifier 7 having a construction example shown in FIG. 7.
The optical amplifier 7 is constituted of a plurality (four in the Figure) of SMFs 1A, an Er-doped optical fiber 2A as an optical amplifier, the light source 4, and an optical coupler 8. Both ends X, X of the Er-doped optical fiber 2A are connected with the two SMFs 1A, 1A respectively.
Here, the SMFs 1A and the Er-doped optical fiber 2A each have different MFD. Therefore, after fusion-splicing the two, it is necessary to apply the heat treatment to the connection points X, X to make their MFDs correspond to each other.
However, the SMFs 1A are usually as short as several meters. This shortness prevents the loss in the connection points X, X from being directly measured accurately. This poses a problem that the time to terminate the heat treatment can not be decided appropriately.
Yet another example is the case of a dispersion compensating fiber module 9 having a construction example shown in FIG. 8.
The dispersion compensating fiber module 9 is constituted of a plurality (two in the Figure) of SMFs 2A, 2A, a dispersion compensating fiber 2B, and two connectors 10, 10 that connect the SMFs. Both ends X, X of the dispersion compensating fiber 2B are connected with the SMFs 2A, 2A respectively.
Also in this case, as the SMFs 2A, 2A are short, it is impossible to directly measure the connection loss in the connection points X, X with the OTDR, as in the case of the optical amplifier 7 shown in FIG. 7.
It is therefore difficult to decide the time to terminate the heat treatment appropriately also in this case. The insufficient or excessive heat treatment might cause the connection loss to be larger than the target value.